[AsmWritter] Fixed "null check after dereferencing" warning

Summary: The 'BB->getParent()' pointer was utilized before it was verified against nullptr. Check lines: 3567, 3581.

Reviewers: jyknight, RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69751